1. Field of the Invention
The present invention relates to the chemical decontamination of contaminants in soil and groundwater in an in situ environment using oxidizing agents.
2. Description of the Prior Art
Conventional in situ treatment technologies for cleaning contaminated subsurface media use injection ports or a combination of injection and extraction ports to deliver reagents and to extract reaction byproducts and contaminants. In situ chemical oxidation requires the delivery of oxidizing reagents in an aqueous medium. Following gravitation, the aqueous reagent solution administered to the subsurface through fixed injection ports becomes an integral part of the groundwater. The volume of contaminated subsurface media in the unsaturated zone above the groundwater table that is affected by the reagent solution is limited to the annular space of the injection ports. Within the groundwater, the reagent solution follows the natural or induced hydraulic gradient. The oxidizing and hydrophilic reagent solution follows preferred pathways, due to physical and chemical heterogeneities of subsurface media. Physical heterogeneities include variability in hydraulic conductivity caused by material changes for example clay versus sand versus gravel soils versus fractured bedrock. Mineral surfaces are hydrophilic. The hydrophilic properties are altered by sorption of organic compounds such as natural soil organic matter and organic contaminants that contain hydrophobic moieties.
The physical limitations of conventional in situ delivery systems, injection wells, and the physical chemical heterogeneities of subsurface media, limit the effectiveness of oxidizing reagent solutions in making contact with contaminants. Moreover, the oxidizing reagents that are typically utilized in in situ chemical oxidation systems, e.g. liquid hydrogen peroxide, permanganate, etc., are unstable and/or short-lived.
Consumption of oxidant by matrix constituents typically exceeds the oxidant consumption by contaminants. To overcome these limitations, large volumes of highly concentrated reagent solutions are typically administered to the contaminated subsurface media. The introduction of highly concentrated and reactive solutions that contain non-specific oxidizing agents poses problems with respect to controlling the progress and the heat of these reactions.
In situ oxidation systems are known that chemically oxidize organic contaminants to environmentally safe and non-toxic constituents. One such system is a reaction named after its discoverer, H. J. H. Fenton (1894). In this reaction, the oxidizing agent, hydrogen peroxide, is reacted with a metallic salt to generate free radicals with a higher oxidation potential than hydrogen peroxide. The free radicals react with organic compounds to either completely decompose them to carbon dioxide and water or to convert them to water soluble and biologically degradable compounds. A drawback to this process is that the catalytic decomposition of hydrogen peroxide and oxidation of organic compounds by radicals are both exothermic reactions.
A number of patents teach the art of treating contaminants with Fenton-type chemical systems in in situ environments. The patents by Brown et al., U.S. Pat. No. 4,591,443, Vigneri, U.S. Pat. No. 5,520,483, Wilson, U.S. Pat. No. 5,611,642, Kelly et al., U.S. Pat. No. 5,610,065, and Cooper et al., U.S. Pat. No. 5,967,230, teach the introduction of liquid hydrogen peroxide and a metal catalyst, Fenton's Reagent, such as an iron salt, into the subsurface. Watts et at., U.S. Pat. No. 5,741,427, teaches the injection of a chelated metal catalyst for use in an in situ chemical oxidation. All of the above cited art adds a metal catalyst into the subsurface. In addition, the processes described in the above cited art include either the co-injection or the sequential introduction of reagents, where the oxidizing agent is added either before or after the metal catalyst. Finally, all of the prior art teaches the necessity of introducing both the oxidizer and the metal catalyst separately into the subsurface to facilitate the oxidation of contaminants.
It would be of benefit to the art if the use of metal catalyzed peroxides to oxide underground contaminants could be improved whereby it could be simplified, be more controllable, and would produce good results without large amounts of exothermic heat being generated.